Please, Cut Down That Tree: A Solution To Urban Chaos

Trees make us feel better, but they have a cost, sometimes an enormous cost, in terms of road safety and management. Dispassionate analysis should guide policy decisions, which suggests some trees should be cut down.

Rahul Tongia
Fellow at Brookings India and Adj. Professor at Carnegie Mellon University

"Water, water, everywhere, nor any drop to drink. "This lament of a mariner seems to aptly describe many trees I see in urban areas, especially Bangalore, the "Garden City". Bangalore has wonderful green spaces, including two large historical gardens in the middle of the city (Lalbagh and Cubbon Park). But many of the spectacular new gardens I see are not in public spaces but private complexes and townships. I do see a lot of trees, but many are in the wrong place, in the middle of the road. Well, not middle, which wouldn't be so bad (the median), but towards one side of the road, cutting the lanes down in size or even number.

Urban areas and vanity trees

We all instinctively like trees. They give us shade and food (some of them), and are easy on the eye. We are also taught they give us oxygen, and get rid of carbon dioxide. True, but only during the day, during photosynthesis. More importantly, the act of absorbing carbon dioxide and storing carbon in the tree happens primarily when the tree is growing -- once the tree matures there is very little net carbon dioxide absorption. For many of the benefits from trees, we have to understand what are local benefits versus global benefits, i.e., these would still come from a tree far away from the city, for example, in a forest.

Trees don't absorb many pollutants, instead acting as physical barriers, so they could create canopies along roads making some areas better off, but others worse.

Why would I even consider a tree in a forest or plantation superior to an urban tree? A single tree is unlikely to support any wildlife or ecosystem. In fact, many urban trees are stunted and struggle to grow, with their roots paved over, some even dependent on expensive watering by tankers. In contrast, a contiguous group of trees without paving or cement, even urban, can shelter small wildlife, and produce a bio-centric ecosystem.

But what about pollution? There are studies that show that trees can increase local pollution by trapping particulates in an area. Trees don't absorb many pollutants, instead acting as physical barriers, so they could create canopies along roads making some areas better off, but others worse. In addition, some species can actually make smog worse.

Since a growing tree absorbs far more carbon dioxide than a mature tree, if we had to choose to cut down a tree, we could offset this by planting, say, five or 10 trees in other locations.

The cost of city trees

Trees make us feel better, but they have a cost, sometimes an enormous cost, in terms of road safety and management. In one stretch of Bangalore, the trees swoop down onto the road so much they create a risk to drivers, causing multiple accidents, some fatal (sadly, at night, they are not so visible). While we resist cutting trees down, even trimming them isn't done enough for road safety or to isolate electrical wires. Issues of trees are far worse in some cities and neighbourhoods than others, given we haven't planned or synergised co-habitation of people/roads with trees.

A single tree that only causes a mild slow-down in speeds (braking, etc.) wastes lakhs of rupees of petrol...

More than direct safety from hitting the tree, drivers face congestion and vehicle-to-vehicle accidents due to trees. If we do very, very simple order of magnitude estimation, if there is a single tree blocking part of a lane, or even eliminating a lane for a short stretch, this sort of choke-point creates immense traffic jams (two lanes shrinking to one and then back leads to inevitable congestion). A single tree that only causes a mild slow-down in speeds (braking, etc.) wastes lakhs of rupees of petrol (assuming 5000 cars or car equivalents per day). This is before we add in wear-and-tear, value of people's time, etc. Now, if we have a major chokepoint, the delays (in the order of a minute-plus instead of 20 seconds from the earlier example), it can cost 10x more per year just in petrol. That is before we apply a carbon cost to the wasted petrol.

Cost-benefit analysis to aid decision-making

We shouldn't cut down trees on a whim, in the guise of "development". However, one can apply a holistic (societal) cost-benefit analysis to figure out where trees are worth cutting down, to be offset (from a carbon perspective) with new plantings elsewhere. Societal cost-benefit analyses are very hard to do well, since we have to combine different aspects (or units) of value onto the same metric, which will always be fraught with assumptions. This is before the challenge of comparing different stakeholders, such as transient drivers versus the local neighbourhood. Nonetheless, this is still superior to a narrower Return on Investment (RoI) type calculation which is only based on a single ("investor") perspective.

[O]ne can apply a holistic cost-benefit analysis to figure out where trees are worth cutting down, to be offset (from a carbon perspective) with new plantings elsewhere.

A simple cost-benefit calculation would be the locations where the wasted carbon from petrol is greater than the carbon value of the tree. Even where there is a net positive carbon value to keeping the tree, we could simply plant trees elsewhere, with economic savings based on value of petrol saved, not to mention time which leads to productivity.

A carbon calculator shows that a single small/mid-sized car needs several trees to offset annual driving, and multiple times more to be planted to ensure at least one makes it to maturity (40 years of growth phase). Assuming 10 trees, that is only a few hundred rupees to go plant them. If we think that is a cop-out, what if we added in a tax to cover the land for these 10 trees? At 20x20 feet per tree, that is 4000 sq ft of land for the 10 trees, so about 0.1 acres. Even at Rs. 1 lakh/acre (prices relevant only to areas far from cities), that's still a manageable Rs 10,000 to make driving the car virtually carbon neutral, far lower than the taxes and registration for a vehicle. In the long run, vehicles will become electric, which will be more efficient and even lower carbon as the power grid adds more renewable energy.

This note isn't a manifesto to cut down trees, but rather a wish that we can be more holistic in our thinking, urban planning, etc. More gardens should be established, but with better planning, such as creating a buffer zone between homes and traffic. The point is that many things aren't black-and-white -- there are assumptions, nuances, and trade-offs.

More gardens should be established, but with better planning, such as creating a buffer zone between homes and traffic.

Dispassionate analysis could lead us to surprising conclusions. Helipads on roofs -- mandatory on buildings over 60m, are pretty much useless as of today, and certainly unlikely to be more cost-effective than proper fire exits, extra tall ladders, etc. Similarly, many buses in Bangalore aren't the best planned. Very large buses in narrow lanes regularly choke traffic, causing jams lasting a long time. Maybe smaller buses are better -- with higher occupancy they could also afford us to run more often. But fixing transportation is a topic for another day. For now, let's fix some roads by (selectively) cutting down those trees.